Natural Antacid Helped Early Land Creatures Breathe

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The earliest creatures to crawl out of the water onto land may
have concocted antacids out of their own bones, a clever
innovation that would've let the animals breathe, researchers now
find.

The
earliest tetrapods, or four-limbed creatures, made their
first evolutionary forays onto land about 370 million years ago.
Breathing air came with challenges, though. A major one was
getting rid of the air's carbon dioxide, which, when it builds
up, reacts with water in the body and forms an acid.

Now, growing evidence in modern reptiles suggests that bones that
grew within the skin of early tetrapods may have acted as a
natural antacid by releasing their neutralizing chemicals into
the bloodstream. The result would have bought the creatures time
to spend on land before they had to head back to the water to rid
themselves of excess carbon dioxide.

"Now we know that dermal bone can do this and it's something we
didn't know before, that gives us a basis that maybe this is why
tetrapods had this feature, which previously we didn't have a
good explanation for," study researcher Christine Janis, a
paleontologist at Brown University, told LiveScience. "It's the
discovery of this new feature of the physiology of these living
animals that lets us go back [in time]."

First on land

So let's rewind the clock: The first tetrapods evolved from fish
in the Devonian period, which spanned from about 416 million
years ago to 359 million years ago. These early tetrapods had
broad faces, not unlike frogs, and rather immobile ribcages. That
means they wouldn't have been able to get rid of extra carbon
dioxide by breathing quickly, as humans and other mammals do with
their longer snouts and flexible ribcages. Nor were the tetrapods
small enough to exchange carbon dioxide and oxygen via their
skin, as modern amphibians do. [ Top
10 Useless Body Parts ]

What tetrapods did have was complex "dermal bone," or bone that
forms from connective tissue in the skin instead of from
cartilage like the long bones of the arm or leg.The concept of
skin bone may seem strange, but it's very common: The
human skull, for example, is a dermal bone.

Early tetrapod bone showed many pits and furrows, indicating lots
of blood supply, Janis said. Her colleagues, including paper
co-author and biologist Daniel Warren of Saint Louis University,
had found another piece of the puzzle: In modern turtles and
alligators, this dermal bone helps the reptiles tolerate carbon
dioxide buildup when they're under water, unable to breathe.

Bone breathing

Tetrapods would have the opposite problem, Janis realized: They'd
be able to release carbon dioxide through their skin while in the
water, since their skin was more permeable than an
alligator's tough hide. But out on land, they'd need another
means of release. It seemed very possible that tetrapods could
have used their complex dermal bones as a storage unit for
calcium and other acid-neutralizing minerals, releasing them as
needed when body acid levels got too high, Janis said.

To test the idea, the researchers analyzed the skeletons of
tetrapods. As you might expect, the tetrapods known by the
skeletons to spend more time out of the water had the most
complex dermal bones. The evolutionary history of the animal
supports the hypothesis, as well.

"When [the dermal bone] gets lost, it gets lost in the lineage
leading to modern reptiles when they start getting more mobile
ribs," Janis said.

She and her colleagues reported their work Tuesday (April 24) in
the journal Proceedings of the Royal Society B.

End of the early tetrapods

While the evidence is consistent with Janis' theory, there's no
proof yet that tetrapods really used their bones in this way. The
next step, Janis said, will be to look for chemical or other
clues in modern reptiles who use their bones as antacid. If any
telltale signs are established, researchers can then hunt for the
same signals in ancient tetrapods.

The terrestrial tetrapods studied by Janis and her colleagues
went extinct during the
Permian period 299 million to 251 million years ago. It was a
changing world, Janis said, and atmospheric carbon dioxide was
increasing. It's possible that tetrapods' bone-dependent
breathing wasn't as effective in this new atmosphere.

"Who knows?" Janis asked. "I think the point to make is that this
was probably a perfectly good way to live for awhile — millions
of years — but in the end, there were things that had figured out
better ways of how to get rid of carbon dioxide."